1. Field of the Disclosure
This invention pertains to a method for making a cylindrically-shaped element, and particularly a cylindrically-shaped element for use as a base sleeve for printing or as a support for a print form.
2. Description of Related Art
In flexographic printing operations, flat, flexible plates can be hand-mounted onto print cylinders by wrapping and adhering the plates to the underlying cylinder. Generally, the flat plate includes a base support having either a rubber layer with relief indicia or a photocurable polymer layer thereon. In some instances, a compressible layer is positioned between the base support and rubber or photocurable layer to improve print quality. Such flat plates have the advantage that they could be relatively thin and flexible because they directly mounted to the print cylinder. However, such mounting processes are labor intensive and slow, and plates can not be easily removed from the print cylinder for reuse in a subsequent print run.
Hollow cylindrical sleeves have served as supports for various print forms. In some cases a print form consists of printing plate/s mounted to the cylindrical sleeve. In other cases a print form consists of a continuous layer of photopolymer or rubber, which can be imaged, applied to an exterior surface of the cylindrical sleeve. A cylindrical sleeve may sometimes be referred to as a base sleeve. There are particular advantages to using print forms having the continuous layer of an imageable photopolymer or rubber on a cylindrical sleeve. Continuous print forms have applications in flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper.
The use of cylindrical sleeves as well as continuous print forms is becoming increasingly more common in the industry. Unlike plates, sleeves are not adhered to the print cylinder and thus allow for the capability to easily reuse print forms for subsequent print runs. Sleeve technology also permits very rapid and simple changing of the print form on a print cylinder. The internal diameter of a cylindrical sleeve corresponds to the external diameter of the print cylinder so that the sleeve can be simply slid over the print cylinder of the printing press. The print cylinder is equipped with compressed air for facilitating the mounting and de-mounting (i.e., pushing on and pushing off) of the sleeve onto and from the print cylinder. Compressed air is connected to the print cylinder which passes into the interior of the cylinder and emerges via holes arranged on the exterior surface of the cylinder to create an air cushion for mounting and de-mounting the sleeve. For mounting a sleeve, compressed air emerges at the surface holes of the print cylinder and the sleeve is pushed on the exterior surface of the print cylinder creating the cushion of air that substantially reduces the friction between the sleeve and the print cylinder. Since the sleeve expands slightly under the influence of the air cushion, the sleeve easily slides along the print cylinder to the desired position. When the compressed air is terminated, the sleeve no longer can stay expanded and contracts to reside firmly on the print cylinder. However, cylindrical sleeves need to withstand the rigors of mounting and de-mounting from the print cylinder with compressed air.
Thin-walled cylindrical sleeves, that is, sleeves having a wall thickness of about 0.050 inch or less (0.127 cm or less), have particular advantages due to low manufacturing costs, increase production, and ease of use due to their low weight and flexibility. Thin-walled cylindrical sleeves can also easily be mated with bridge sleeves to attain the desired repeat of a printed image with existing print cylinders, and with cushion sleeves to attain suitable print quality, for example, reduced dot gain. Various configurations of cylindrical sleeves are known from U.S. Pat. Nos. 4,214,932; 5,383,062; 5,468,568; 5,753,324; 5,974,972; 6,699,548; and 6,703,095. Generally these prior art sleeves consist of a plurality of associated concentric layers, and perhaps one or more underlying support layers. These known cylindrical sleeves however exhibit a number of constraints with respect to their manufacture and use.
Problems sometimes arise with cylindrical sleeves that are produced with seams where ends or edges of material are joined to form the sleeve. Production of sleeves with seams is particularly difficult since the edges of the material forming the seam need to have sufficient contact to assure an air-tight seal for the sleeve, but yet not have a buildup or allowance of excess material at the seam that can impact print performance by the print form. It is particularly time- and labor-intensive to produce sleeves having seams with edges that sufficiently abut with no or only minimal allowance and contrary toward a goal of manufacturing high volumes of cylindrical sleeves in a plurality of sizes (based on diameter as well as axial length of the cylinder). A sleeve may break or separate at a seam under the application of pressurized air when mounting and de-mounting of cylindrical print forms, thereby rendering the print form inoperable. The seams may not fully form and result in a lack of air tightness necessary for proper mounting of the sleeve onto a print cylinder. Buildup or excess seam allowance of material at the seam results in non-uniformities in the sleeve which can transmit through the one or more layers disposed on the exterior surface of the sleeve, such as the relief image layer, and result in a printing defect, which is sometimes referred to as “print through”. Print through manifests as repeating distortion/s or disturbance/s in the image printed on the substrate, which correspond to the underlying seam structure in the print sleeve. Print through of a seam to the printed image can exhibit a region of slightly higher or lower density of the image compared to the image printed by the remaining, i.e., non-seam parts, of the sleeve. A seam that is thicker or thinner or has different characteristic response under impression than the remaining non-seam portion of the sleeve generally will print through to the printed image. Also, a poorly-formed seam can interfere with the application of one or more layers, such as a photopolymeric layer, onto the exterior surface of the sleeve. Sometimes, the exterior surface of the sleeve is ground to provide desired uniformity of the wall thickness at the seam and for the remainder of the sleeve. However, grinding the surface introduces an additional step in a process of making the sleeve which, for costs and production purposes, is desirable to avoid.
So a need arises for a cylindrical sleeve that is easily and quickly produced, at a low cost, while avoiding the problems of prior cylindrical sleeves. It is desirable to form the cylindrical sleeve in one or more layers from sheet material, and yet avoid defects associated with seam non-uniformities, such as print through, that can be observed in an image printed by the print form, and facilitate the application of additional layers onto the sleeve. The cylindrical sleeve should be capable of supporting print plate/s or a continuous layer of an imageable photopolymer or rubber and withstanding the rigors of mounting and de-mounting with pressurized air onto a print cylinder. The cylindrical sleeve should be capable of maintaining dimensional stability and tolerances during subsequent manufacturing steps, such as formation of the continuous layer thereon, and/or undergoing imaging and treating steps, such as solvent washout or heating, to form a relief surface of the continuous layer that is suitable for printing.